The invention relates to a liquid crystal display device comprising pixels and electrodes for driving the pixels, each pixel comprising a display element defined by picture electrodes, which display element comprises layers of orienting material and a layer of liquid crystal material between the picture electrodes.
Such liquid crystal display devices are generally known and used, for example, in monitors, but also in portable applications (organizers, mobile telephones).
A known phenomenon in such liquid crystal display devices is the displacement of ions in the liquid, so that degradation occurs, which becomes manifest as image retention. To prevent this, liquid crystal display devices are driven with an inverting or alternating voltage across the pixels. This is notably detrimental in portable applications because the use of an inverting voltage is accompanied by a high energy consumption and a high battery voltage for the drive electronics. This in turn leads to higher costs.
A liquid crystal display device according to the invention is characterized in that, for the quotient Q of a dielectric constant ∈LC of the liquid crystal material and the dielectric constant of the layers of orienting material ∈ol, it holds that Q=∈LC/∈ol greater than 0.7 xcfx81ol/xcfx81LC, in which, for a liquid crystal material having a negative dielectric anisotropy (xcex94∈ less than 0) ∈LC, the dielectric constant perpendicular to the directors of the liquid crystal material is (∈xe2x8axa5), and for a liquid crystal material having a positive dielectric anisotropy (xcex94∈ greater than 0) ∈LC, the dielectric constant parallel to the directors of the liquid crystal material is (531∥) and xcfx81ol and xcfx81LC are the resistivities of the liquid crystal material and the layers of orienting material, respectively.
The orientation layer may comprise sub-layers of different material. In that case, xcfx81ol is understood to mean the average resistivity of the orientation layer.
The value Q is preferably between 0.4 and 4, while values of between 1.2 and 3 look optimal.
To inhibit image retention even further, a first embodiment is characterized in that xcfx81ol/xcfx81LC less than 10 (and preferably less than 5) at 25xc2x0 C. To prevent lateral conduction in the orientation layers, xcfx81ol is chosen to be  greater than 107 ohmmeter (T=25xc2x0 C.).
A further embodiment is characterized in that the liquid crystal display device comprises means for presenting drive voltages in one polarity across the pixels (DC drive). In this connection, one polarity is understood to mean that no measures have been taken to change the polarity across the pixels during operations over a longer period of time (e.g. 1000 or 2000 frame times), but measures may be taken to drive the pixels with opposite polarity when modes are re-used or changed (for example, switching from or to a standby mode in a display in a portable application or switching between use of modes, Internetpages etc. in computer applications) of the display device.
The inventors have surprisingly found that the display device can be DC-driven without degradation or image retention occurring in said combination of dielectric constants of the orienting material and of the liquid crystal material. By approximation, a theoretical explanation can also be given for this effect. Since the display device is now DC driven, the electronics for continuously reversing the voltage across a pixel may be dispensed with. (Reversing can be restricted to e.g. once every minute or five minutes). Moreover, there is no flicker.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.